Fuel cells have generated a lot of attention recently due to increased demands for efficient and clean electricity generation. A major inhibitor to mass commercialization of fuel cells is cost. AMFCs offer a promising solution to reduce the cost of effective fuel cells. Alkaline membrane fuel cells are similar to PEM fuel cells but the membranes are designed to transport hydroxide ions instead of protons. One of the biggest cost factors of PEM fuel cells is their dependency on a precious metal such as platinum, which is used as both the air cathode and hydrogen anode catalyst. Platinum is an extremely rare metal, occurring as only 0.003 ppb in the Earth's crust, and is 30 times rarer than gold. Reducing the amount of platinum required (and thus cost) has been a major focus of PEM fuel cell research. Alkaline membrane fuel cells allow elimination of Pt altogether from the fuel cell catalyst, relying on the use of non-precious metal catalysts, which can greatly reduce the cost of fuel cells. The milder electrochemical environment of the alkaline membrane, that allows replacing precious metal catalysts by much less expensive metal catalysts, also allows using stack hardware of much lower cost and superior properties.
Several reports on building and testing of AMFCs on a laboratory scale, have described cell testing with aqueous electrolyte, typically aqueous KOH, continuously added as part of the fuel feed stream, found necessary to achieve reasonable performance. AMFC Performance without any added aqueous electrolyte has been found to be at least an order of magnitude lower than that obtained with proton conducting membrane fuel cells which, as a rule, do not use any added electrolyte. Once liquid alkaline electrolyte is added, a polymer electrolyte fuel cell loses some key advantages of this family of cells, including the avoidance of electrolyte management issues and maintenance of a safe exhaust stream which consists of water vapor alone in the case of hydrogen fuel. As long as it is a prerequisite for obtaining acceptable AMFC performance, continuous liquid alkaline electrolyte addition seriously diminishes the practical value of AMFCs.
There is a significant challenge, however, to the development of optimized electrodes and membrane-electrode assemblies (“MEAs”) specifically designed for AMFCs which do not use any added liquid electrolyte. The challenge has primarily to do with the limited conductivity of OH— ion conducting polymers demonstrated to date, requiring judicious choice of recast ionomer material for the catalyst “ink” and a catalyst layer thickness and structure optimized for minimal transport limitations at the highest demand current.
When looking at the much more developed technology of PEM fuel cells as possible source of information, it is very important to recognize some substantial differences between the technology based on alkaline membrane and the technology based on an acidic membrane. Not only is the ionic conductivity of the alkaline ionomer substantially lower, the sides of the cell where water is generated and consumed are reversed. In the AMFC, water is generated on the fuel side and consumed on the oxygen, or air side. This creates AMFC technical challenges of special nature that are not shared with the PEM counterpart. Product water removal without loss of fuel becomes an important issue as result of the water generation at the fuel electrode. Furthermore, cathode dry out, resulting in strong cathode performance loss, is a clear danger in the AMFC, because water is being consumed at the cathode, rather than generated there as in the case of the PEM cell, and the active flow of air by the cathode would tend to carry with it any cathode water out the cathode exhaust. Water management therefore has a different problem set and a higher degree of severity in the AMFC.
Fuel cells are often operated with pure hydrogen gas as the fuel. When using pure hydrogen as a fuel, a PEM fuel cell can be operated in what is known as a “dead-end” anode configuration, which means the anode compartment has an inlet, but no outlet. Design of a fuel cell based on a dead-ended anode, has been recognized as having valuable advantages, including system simplicity, zero fuel emissions and high fuel utilization. However, in an AMFC, water is a product that is generated in the anode side of the cell. Consequently, a dead-ended anode in an AMFC requires that there be a way to continuously remove excess product water from the anode. Product water removal from an operating AMFC through an open-ended anode could mimic the established mode of water product removal from fuel cells based on proton conducting membranes, where water is generated at the air cathode and leaves through the exhaust of the open-ended cathode. Such straightforward solution for excess water removal will, however, result, in the case of the AMFC, in significant fuel loss at high hydrogen flow rates, or poor utilization of active electrode area when the rate of fuel supply is lowered sufficiently to avoid fuel loss. In summary, AMFC area power densities significantly above 100 mW/cm2, while using non-Pt catalysts and avoiding added liquid electrolyte, has not been described to date. Also, effective AMFC water management in general, and particularly water management without fuel loss, the case of an AMFC configured with a dead-ended anode, have not been described to date.